Mechanism of Inhibition of <i>Bacillus anthracis</i> Spore Outgrowth by the Lantibiotic Nisin

The lantibiotic nisin inhibits growth of vegetative Gram-positive bacteria by binding to lipid II, which disrupts cell wall biosynthesis and facilitates pore formation. Nisin also inhibits the outgrowth of bacterial spores, including spores of <i>Bacillus anthracis</i>, whose structural and biochemical properties are fundamentally different from those of vegetative bacteria. The molecular basis of nisin inhibition of spore outgrowth had not been identified, as previous studies suggested that inhibition of spore outgrowth involved either covalent binding to a spore target or loss of membrane integrity; disruption of cell wall biosynthesis <i>via</i> binding to lipid II had not been investigated. To provide insights into the latter possibility, the effects of nisin were compared with those of vancomycin, another lipid II binding antibiotic that inhibits cell wall biosynthesis but does not form pores. Nisin and vancomycin both inhibited the replication of vegetative cells, but only nisin inhibited the transition from a germinated spore to a vegetative cell. Moreover, vancomycin prevented nisin’s activity in competition studies, suggesting that the nisin-lipid II interaction is important for inhibition of spore outgrowth. In experiments with fluorescently labeled nisin, no evidence was found for a covalent mechanism for inhibition of spore outgrowth. Interestingly, mutants in the hinge region (N20P/M21P and M21P/K22P) that still bind lipid II but cannot form pores had potent antimicrobial activity against vegetative <i>B. anthracis</i> cells but did not inhibit spore outgrowth. Therefore, pore formation is essential for the latter activity but not the former. Collectively, these studies suggest that nisin utilizes lipid II as the germinated spore target during outgrowth inhibition and that nisin-mediated membrane disruption is essential to inhibit spore development into vegetative cells.